The invention relates to an apparatus for determining absolute angular position of a steering wheel of a motor vehicle rotatable over several complete revolutions.
For certain safety systems in motor vehicles (for example automatic rear axle steering, vehicle dynamics control), for safety reasons the current steering wheel position is required immediately after the vehicle is started. In particular, sensors with an incremental mode of operation, in which the angle increments are arranged in only one track, can only fulfill this requirement insufficiently since an index mark must be reached in all cases before the sensor supplies a valid measurement value. Furthermore, such a steering wheel angle sensor must supply a measurement value in the range .+-.720.degree.. This means that a second system must be present within the sensor, which detects the angle values which exceed one revolution. This system must also supply a valid angular position value immediately after the ignition is switched on.
Analog sensor systems, such as potentiometers (Patent Document EP 0 477 653 A1) as such fulfill this requirement, but have the disadvantage that they do not operate in a wear-free manner and in this regard do not fulfill the requirements for a safety system.
Sensing apparatus for digital systems which make absolute measurements (sensing of a plurality of code tracks within one revolution) are too complex and cannot be cost-effectively accommodated by known means in the existing physical space, or only with difficulty (Patent Document EP 0 489 350 A1).
DE-P 42 20 883 discloses a steering angle sensor in which two fine signals are generated, which are shifted with respect to one another and are repeated every 360.degree.. Furthermore, a coarse signal is generated which extends over an angular range of four revolutions of the steering wheel and makes it possible to identify which revolution the steering wheel is in.
Another known absolute angle sensing apparatus is based on a single-track incremental code which, in conjunction with a plurality of sensors, supplies an absolute value within one revolution of the steering wheel (Patent Document EP 0 377 097 B1). However, this latter angular position sensing apparatus cannot detect steering wheel angles which exceed 360.degree.. This disadvantage has been previously remedied by suitably storing the last valid angular position value, including the number of steering wheel revolutions (usually by means of an electronic memory) after initializing the angle measurement system and by retrieving and using this stored value when the vehicle is started again. A necessary condition for this is that the position of the steering wheel no longer changes after the ignition is switched off. However, since this cannot be ruled out in all cases, the sensor is often not disconnected from the electrical system of the vehicle after the ignition is switched off, so that changes in the angular position of the steering wheel can be detected even after the ignition is switched off. In such cases, the sensors are switched to a standby mode, so that the battery of the vehicle does not discharge too quickly. There is still always the risk of the supply voltage failing, because the sensing apparatus then loses its stored information regarding the number of revolutions of the steering wheel in such cases. After such events, the sensor must be initialized again by means of a special procedure, which as a rule can only take place in special workshops.
It would therefore be desirable to have an apparatus for determining absolute angular position of a motor vehicle steering wheel which would supply the absolute angle within the steering wheel angular travel range of .+-.720.degree. immediately when the ignition is switched on. A purely digital method of operation of the sensor within the entire steering wheel angular range would also be desirable to avoid complex adjustment procedures.